Soft foams and elastomers that have low resilience and recover slowly when compressed are referred to as viscoelastic. These materials are well known in the art and are useful for their energy absorbing properties. Viscoelastic foams and elastomers are used in a variety of applications to impart cushioning (such as in pillows, wheelchair seats, mattresses, etc.), as sound and vibration damping materials, to impart impact protection, etc. Reactively processable polyurethanes and polyurethaneureas have been particularly useful for the production of viscoelastic materials. Polyurethane chemistry offers unique opportunities for the design of materials with finely tuned physical properties, and for the in-situ fabrication of complex shapes and composite structures.
Reactively processed polyurethane and polyurethaneurea materials are usually made by the combining of two or more liquid chemical streams. The mixing of these liquid streams initiates the polymerization, and, where appropriate, foaming reactions. Shaping and polymerization often occur in the same operation, typically by molding or spraying of the combined reaction mixture while it is still in a liquid state. Polyurethane and polyurethaneurea materials may also be prepared in bulk and then cut into shapes or otherwise post formed after polymerization. A very widely used method for reactive processing of polyurethanes and polyurethaneureas is the two stream approach wherein one stream is a polyfunctional organic isocyanate component (sometimes referred to as the “A-component”) and the second stream is a mixture of polyfunctional isocyanate reactive monomers and resins containing catalysts and other additives (sometimes referred to as the “B-component”). The two streams are metered and mixed at a desired weight ratio, determined in part by the stoichiometry of the reaction. Foaming is usually accomplished by including water in the B-component. The water reacts with the polyisocyanate to liberate CO2 and form urea linkages in the polymer, thereby forming a polyurethaneurea polymer structure. Volatile inert organic compounds or inert gasses are sometimes used, either alone or in combination with water, to achieve foaming. Many variations of this basic process are known. Some involve the addition of the water (or other blowing agents), catalysts, and/or other additives as additional chemical streams during processing. Still other known variations involve the physical injection of inert gasses after the point at which the reactive chemical streams are mixed, but before gelling occurs, in order to promote foaming. The degree of foaming may, of course, vary considerably depending upon the level of blowing agent(s) used. Relatively high density elastomeric polyurethane and polyurethaneurea foams are sometimes referred to as microcellular elastomers. Water is also the preferred blowing agent for preparing microcellular elastomers, although the use of water is often supplemented with dissolved or dispersed atmospheric gases such as air or nitrogen.
Most conventional polyurethane and polyurethaneurea solid elastomers, microcellular elastomers, soft foams, and semiflexible foams are phase separated block copolymers comprising distinct phases with high and low glass transition temperatures (Tg's). These high and low Tg phases usually bracket the use temperature range of the material, and the dynamic mechanical spectra of these materials usually show the distinct glass transitions separated by a relatively flat region (or modulus plateau). The low Tg phase in these materials is usually (although not always) derived from a low Tg “block” which is pre-formed and incorporated via the reactive liquid chemical streams. The high Tg phase, by contrast, usually forms during polymerization, due to the formation of urethane and/or urea linkages. The low Tg block (often called the “softblock”) is usually derived from a liquid or low melting oligomeric resin that contains a plurality of groups reactive with isocyanate groups. Polyether and polyester polyols are typical examples of these oligomeric resins. In conventional elastomeric polyurethanes and polyurethaneureas the hard (high Tg) and soft (low Tg) phases self assemble during the polymerization reaction and then spontaneously segregate into morphologically distinct phases within the bulk polymer structure. They are said to be “phase separated” materials.
Viscoelastic polyurethanes and polyurethaneureas form a subclass of elastomeric materials and semiflexible materials, characterized by having relatively incomplete or non-existent phase separation. These materials are sometimes referred to as “phase mixed”. The phase mixed morphology is characterized by having a single large glass transition, usually within the normal use temperature range of the material (which is typically from about 0° C. to about 50° C.). There may also be additional glass transitions in the dynamic mechanical spectra of these materials, but these, where they exist at all, are much smaller than the main transition. The main glass transition in a phase mixed material is generally quite broad, covering all or most of the normal use temperature range of the material. This main transition accounts for the unique energy absorbing (low resilience) properties that are characteristic of viscoelastic polyurethanes and polyurethaneureas. Block polyurethanes and polyurethaneureas that are well phase separated are, by contrast, highly resilient and have poor energy absorption properties within the modulus plateau region (between the distinct hard and soft phase glass transitions).
The most effective methods for achieving viscoelastic properties in polyurethane and polyurethaneurea materials involve interfering with the phase separation of the polymer segments in some way. These methods, in effect, blur the interface between the hard and soft blocks. Some specific methods that have been used in the art include relatively heavy crosslinking, reducing the equivalent weight of the soft segment precursor (in effect, reducing the length of the elastically active chains), using mixtures of soft segment precursors (polyols) comprising relatively low, medium, and high equivalent weight components, the addition of high levels of plasticizers, the addition of high levels of particulate fillers, and various combinations of these methods. Each of these prior art methods has undesirable tradeoffs, especially when relatively soft materials are required: Excessive crosslinking can severely reduce important mechanical properties, such as elongation, tear resistance, and tensile strength. It may also cause high compression sets and undesirably high hardness. Reduction of the length of the elastically active chains and the use of polyol mixtures with high/medium/low equivalent weights may also cause excessively high compression sets. It is important that a viscoelastic material be capable of recovering its original shape (albeit slowly) when compressed or otherwise distorted. It should not take a permanent set. Therefore, low compression set, both under humid aged and dry conditions, is a valuable physical property characteristic for viscoelastic polyurethanes and polyurethaneureas. Finally, the use of high levels of plasticizers or fillers also detracts from mechanical properties related to strength, such as tensile strength and tear resistance. Very high levels of these additives, typically well over 30% by weight, and often more than 100% of the weight of the polymer, are required in order to achieve useful viscoelastic performance. Filler loadings this high can result in excessive hardness and weight. Plasticizers, when used at such extreme levels, can migrate and cause staining as well as toxicity concerns and odor problems. Viscoelastic polyurethane and polyurethaneurea foams should be predominantly open celled in order to avoid shrinkage. Some of the known techniques used to promote viscoelastic behavior, such as the use of polyols with low to medium range equivalent weights, often interfere with the polyurea (hard segment) precipitation or micro-domain formation. This can in turn interfere with spontaneous cell opening. Although it is sometimes still possible to achieve cell opening by crushing the foam, this is not always practical and it adds a costly extra step to the process.
There is a noticeable lack of prior art references to flexible polyurethane or polyurethaneurea foam formulations based predominantly on combinations of PO-rich and EO-rich flexible polyols of intermediate equivalent weights. PO-rich polyols are generally regarded as hydrophobic, whereas EO-rich polyols are considered relatively hydrophilic. Polyols that are very high in PO may be incompatible with polyols very high in EO, such that mixtures of these polyols often phase separate. It is particularly difficult to process such mixtures into flexible or semiflexible foams, in as much as the incipient foams have a strong tendency to collapse.
Therefore, there is a need in the industry for improved soft and also semi-rigid (semi-flexible) viscoelastic polyurethanes and polyurethaneureas that are reactively processable, have low compression set values, good strength and elongation properties, and do not require the use of high loadings of plasticizers or fillers. There is a further need for reactively processable viscoelastic polyurethane and polyurethaneurea foams which, in addition to having all the characteristics noted above over a wide range of foam densities, are predominantly open celled, cover a very wide hardness range, and do not require crushing.